Parabolic trough solar concentrators use mirrored surfaces curved in a parabolic shape. The mirrors focus sunlight on a receiver tube, or heat collection element (HCE), running the length of the trough. In a trough power plant, oil runs through the HCE in the focal region where it is heated to high temperatures and then goes through a heat exchanger to generate steam. The steam is then used to run a conventional power plant. Such parabolic trough solar concentrators have been developed, fielded, and are currently producing electricity in the United States and are in development in other nations. See H. Price et al., “Advances in Parabolic Trough Solar Power Technology,” ASME J. of Solar Energy Engineering 105, 288 (2002); and V. E. Dudley et al., “Test Results: SEGS LS-2 Solar Collector,” SAND94-1884, Sandia National Laboratories, Albuquerque, N. Mex. (1994).
Most trough concentrators use multiple mirror facets that have to be aligned to the HCE. Accurate mirror alignment of faceted solar concentrators maximizes the reflected sunlight intercepting the HCE and can enable the use of a smaller HCE, or the use of a larger aperture concentrator with the same size HCE, thereby improving overall collector efficiency. In addition, practical alignment can potentially reduce solar collector installation-fixture accuracy requirements and cost. However, a problem with trough concentrators has been the lack of accurate mirror facet alignment, preventing maximum energy efficiency.
Compared with parabolic dish concentrators, practical optical alignment techniques for the accurate alignment of parabolic trough concentrators have not been developed. The relatively short focal lengths and low operating temperature in parabolic trough systems have allowed them to be developed and commercialized with relatively inaccurate alignment by the use of fixtures. Their linear nature has also been a barrier to the development of practical optical techniques. Parabolic dishes, on the other hand, require precise alignment, especially to minimize flux hot spots on the solar receiver. In addition, the fact that the facet normals of parabolic dishes conveniently point to the same general location (approximately one focal length behind the dish focus) facilitates alignment.
Accurate alignment of concentrating collectors by the use of fixtures is extremely difficult. Error stack-up and indeterminate effects, such as thermal expansion, make the use of fixtures challenging for precise large optical systems. For parabolic dishes, only optical techniques, which inherently account for error stack-up and other factors, have provided the required alignment accuracy. Where optical techniques have been used to measure the alignment of fixture-aligned parabolic troughs, significant misalignment has been reported. Mechanical fixtures also do not lend themselves to checking alignment after installation.
Various optical techniques have been developed to align parabolic dishes. Distant observer and distant light source techniques have been developed to align parabolic dishes, lasers have been used to align solar furnace mirrors and parabolic dishes, and a video-based technique for mirror characterization and facet alignment has been developed and implemented. See F. R. Livingston, “Activity and Accomplishments in Dish/Stirling Electric Power System Development,” DOE/JPL-1060-82, Pasadena, Calif. (1985); M. K. Selcuk, “Parabolic Dish Test Site: History and Operating Experience,” DOE/JPL-1060-84, Pasadena, Calif. (1995); R. B. Diver et al., “A New High-Temperature Solar Research Furnace,” ASME J. of Solar Energy Engineering, Vol. 105, pp. 288-293 (1983); R. B. Diver, “Method and apparatus for aligning a solar concentrator using two lasers,” U.S. Pat. No. 6,597,709; and J. B. Blackmon and K. W. Stone, “Application of the Digital Image Radiometer to Optical Measurement and Alignment of Space and Terrestrial Solar Power Systems,” Paper No. 93217, Proceedings of the 28th IECEC, Atlanta, Ga. (1993). Variations on the distant light source technique have been further developed to enable near alignment and daylight alignment by the use of color targets and video cameras. See R. B. Diver, “Mirror Alignment Techniques for Point-Focus Solar Concentrators,” SAND92-0668, Sandia National Laboratories, Albuquerque, N. Mex. (1992); R. B. Diver, “Mirror Alignment and Focus of Point-Focus Solar Concentrators,” Solar Engineering 1995, Proceedings of the ASME/JSME/JSES International Solar Energy Conference, Maui, Hi. (1995); C. E. Andraka et al., “Improved Alignment Technique for Dish Concentrators,” International Solar Energy Conference Proceedings, Kohala Coast, Hawaii Island, Hi. (2003); and B. J. Steffen et al., “Development and Characterization of a Color 2F Alignment Method for the Advanced Dish Development System,” International Solar Energy Conference Proceedings, Kohala Coast, Hawaii Island, Hi. (2003). With these techniques, differences between theoretically calculated and optically measured image positions are used to guide alignment.
Despite the relatively advanced state of commercialization of parabolic troughs, optical alignment is undeveloped. One of the early concepts proposed utilizing reflected images in the mirrors, but it was never developed. See R. L. Wood, “Distant Observer Techniques for Verification of Solar Concentrator Optical Geometry”, UCRL53220, Lawrence Livermore National Laboratory, Livermore, CA (1981). The use of lasers to statistically determine optical accuracy and mirror alignment has received the most attention, and an approach based on stereoscopic photography has shown promise. See T. Wendelin (2004); B. L. Butler and R. B. Pettit, “Optical Evaluation Techniques for Reflecting Solar Concentrators,” SPIE Vol. 144 Optics Applied to Solar Energy Conversion, Society of Photo-Optical Instrumentation Engineers, Bellingham, Wash. (1977); E. Lupfort et al. (2005); and M. R. Shortis and G. Johnston, “Photogrammetry: An Available Surface Characterization Tool for Solar Concentrators, Part II: Assessment of Surfaces,” ASME J. of Solar Energy Engineering, Vol. 119, pp. 281-291 (1997). The distant observer technique has been used to align LS-2 trough facets as part of its HCE performance characterization. See T. A. Moss and D. A. Brosseau, “Final Test Results for the Schott HCE on a LS-2 Collector,” SAND2005-4034, Sandia National Laboratories, Albuquerque, N. Mex. (2005). Unfortunately, trough spacing requirements do not permit the use of the distant observer technique within a trough field. The laser and stereoscopic techniques are also complex, require sophisticated equipment and setup, and are impractical for the staggering number of mirrors in a trough solar power plant.
A desirable mirror alignment method for any concentrating solar collector would: (1) be simple to set up and implement; (2) use a minimum of sophisticated hardware; (3) not require removal of the HCE receiver; (4) not require sun or other restrictive weather conditions; (5) not require line-of-sight to a distant observer or light source; and (6) permit accessibility to the mirrors for adjustments. See R. B. Diver (1995). The Theoretical Overlay Photographic (TOP) alignment method of the present invention provides these desirable features for aligning parabolic trough solar concentrators.